Everything seems to go fine with sending emails with NetMailBot - I receive no errors, either from NetMailBot, my sending SMTP server, or any of my recipients' receiving SMTP servers, yet some of my intended recipients still get no email. What is happening?

An increasing "problem" is the use of Unsolicited Commercial Email (UCE, or commonly referred to as "SPAM") filtering. Recall that NetMailBot communicates directly with your sending SMTP server, but the sent emails have to make their way to the recipients' receiving SMTP servers and ultimately to your recipients' mail clients. Along this chain, a mail might be judged as "SPAM" and filtered out, possibly at the receiving server, or at a client program. Unfortunately there is no way to control this from your end. If people who are expecting your mail complain that they never receive it, and you encountered no errors in sending it, they might want to check with their system administrators to see if their receiving mail server is configured for "SPAM" filtering.